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The invention relates generally to motors or drives for micro-electro-mechanical systems (MEMS) devices, and more particularly to inter-digitated electrostatic comb drives.
A variety of drives and motors have been developed for MEMS applications to actuate a variety of devices, such as electronic or fluid switches, or to move an object, such as a mirror or other optical device, from one position to another. For example, MEMS devices have been developed for use in optical switching applications in optical communication systems.
One such device uses a small mirror fabricated on a movable portion of a MEMS drive, the mirror being moved into and out of the path of an optical beam. When the mirror is out of the beam path, the optical beam continues to one destination, and when the mirror is in the beam path, the optical beam is reflected to another destination.
This device uses an electrostatic comb drive actuator with two separate comb drives attached to the movable element (mirror) of the device to achieve bi-directional operation. An example of a MEMS device with dual comb drives is illustrated and described in International Patent Publication No. WO 98/12589, entitled A FIBER OPTIC CIRCUIT SWITCH AND A PROCESS FOR ITS PRODUCTION by Anthamatten et al., published Mar. 26, 1998. One drive has a fixed set of fingers and a corresponding set of interdigitated movable fingers to pull the movable portion in one direction, and a second set of fixed and interdigitated movable fingers to pull the movable portion back in the opposite direction. The device can be cycled by applying an appropriate actuation signal first to one set of fingers, and then applying another actuation signal to the other drive. The device can be latched in either position by a variety of mechanisms, such as by using mechanical latches, magnetic latches, or electrostatic latches. Alternatively, the actuation signal may be continuously applied to retain the switch state of the device.
However, mechanical overshoot and ringing can occur due to the kinematics of the comb drive. For example, an actuating pulse of sufficient voltage and duration to switch the least compliant comb drive may cause the most compliant comb drive to overshoot the desired position due to the momentum of the movable elements. Such overshoot can cause a number of problems. For example, the momentum might cause the generally thin, fragile interdigitated fingers to crash into the opposite wall of the comb drive, or the movable element might first overshoot and then recoil back to the original position.
The mirror might overshoot past where it can reflect the optical beam, temporarily xe2x80x9cdroppingxe2x80x9d the beam until the mirror settles into its correct position. This settling can require an additional period to be specified in the switching time of the device, which is generally undesirable. Such settling is often referred to as xe2x80x9cringingxe2x80x9d, as the device oscillates between one position and another in a decaying fashion. Some devices use a fluid, such as a heavy gas, pressurized gas, or liquid, to damp the ringing more quickly; however these fluids complicate packaging of the device and may be unwanted or even prohibited in some commercial applications because of the change in performance or damage to other devices if the fluid leaks out. In a latching electrostatic comb drive system, overshoot of the movable portion of the device might even cause the movable portion, after overshooting the desired position, to spring back to latch in the initial position.
Another technique used to reduce overshoot and ringing is to apply an acceleration pulse to the comb drive to initiate motion of the movable portion in the desired direction, and then to apply a breaking pulse to the opposing comb drive (in a two-drive system) to decelerate the movable portion. Such a technique is described in copending co-assigned U.S. patent application Ser. No. 09/517,913 entitled METHOD OF ACTUATING MEMS SWITCHES by Hichwa et al., filed Mar. 3, 2000. The braking pulse helps to remove some of the kinetic energy from the system. While effective, the voltages, duration, and timing of the two pulses must be determined and reliably provided.
Thus, an electrostatic comb drive that avoids overshoot and ringing is desirable. It is further desirable that the comb drive be reliably actuated with a relatively simple actuation signal, have high manufacturing yields, and be robust.
The invention provides a bi-directional self-braking electrostatic comb drive. The motive force of the comb drive is used in combination with a mechanical spring to achieve bidirectional operation with a single set of movable fingers and two sets of fixed fingers. The fingers do not have a constant width, as viewed from the top, but rather have thick portions attached to the base or movable portions of the device with thinner portions. When the interdigitated thick portions are aligned, the device is at the electrostatic center as this is the configuration at which the electrostatic attraction is the least.
Attractive electrostatic force draws the halves of the comb drive together from an initial position. The momentum of the movable portion of the drive in combination with latching spring arms carries the movable portion through and past the electrostatic center. The latching spring arms latch the movable portion of the drive in the switched position, allowing electric power to be removed from the device while retaining the switch state. A second pulse again draws the movable portion back toward the electrostatic center, and momentum carries the movable portion to the initial position, where it is latched again.
In a particular embodiment, a latching spring arm provides additional motive force to carry the movable portion through the electrostatic center and to latch the device in one position or the other. In yet another embodiment, a device starting at the electrostatic center with zero momentum (achieved by applying sufficient voltage long enough to hold the center position) defaults to a known position by operation of the latching spring arms. Thus, this device may be initialized to a known state without sensing the routing of the switched signal.
In another embodiment the movable portion of the comb drive has opposing sets of fingers, each set interdigitated with a corresponding set of fixed fingers. The reciprocal nature of the device, namely that a simple, square-wave pulse of the same polarity will toggle the drive, in combination with the latching spring arms provides a compact and efficient MEMS actuator with relatively high motive force.